The invention relates to an improved method and mechanism for pressing water from a traveling paper web, and more particularly to a press arrangement known as an extended press nip wherein the web is subjected to pressing pressures for a longer period of time than the usual arrangement wherein it passes between two opposed press rolls.
More particularly, the invention relates to a method and structure having first and second press nips wherein the residence time of the web in the nips is increased over that of a roll couple and wherein an improved structure is used to extract water from a web. Other structures have been provided heretofore which have attempted to increase the time over which a web is subjected to a pressure, and yet permit the web to continue movement at a speed necessary in a high speed paper making machine. Such structures have met with degrees of success and are exemplified by the disclosures of U.S. Pat. Nos. 3,748,225, Busker et al; 3,783,097, Justus; 3,797,384, Hoff; 3,798,121, Busker et al; 3,804,707, Mohr; 3,808,092, Busker; 3,808,096, Busker et al; 3,840,429, Busker et al; 3,853,698, Mohr. The devices and method discussed by these previous patents have taken advantage of the knowledge that the static application of mechanical pressure to wet paper mat can reduce the moisture content in the mat to below 40%. Under the dynamic short-term mechanical pressing which occurs in the usual paper machine where the web is run between a series of nips formed between press roll couples, it is often difficult to maintain moisture levels below 60%. Attempts to obtain increased dryness in the conventional roll-couples are usually made by increase in the press nip pressure, but a plateau is soon reached where major increases in roll loading result in relatively small decreases in moisture.
As is known, it is far more efficient to remove water in the press section of a paper machine than in the thermal dryer section and significant reduction in energy costs and significant reduction in the space needed for the dryer drum section of the machine are achieved for every fraction of a percent of moisture that can be additionally removed in the press section. The difficulty of removing moisture in the press section is increased with increase in machine speed because limiting factors are reached in press nip pressures in that compacting and crushing of the web results with higher nip pressures and resultant higher hydraulic pressures within the paper mat. The most feasible way that has been discovered to increase water removal at high speeds has been to increase the residence of pressure time to allow more time for flow to occur within the paper mat and for the hydraulic pressure to dissipate and for water to be pressed out of the web into the felt.
It is accordingly an object of the present invention to take advantage of the principles of extended nip pressing which increases the time that a web is subjected to pressing pressure and to provide an improved method and mechanism utilizing these principles which more effectively removes water from the web increasing the dryness of the web leaving the press section and reducing the possibility of crushing and improving the overall quality of the web formed in a paper making machine.
A further object of the invention is to provide a unique and improved extended nip press which is capable of being constructed for very wide paper machines reducing the problems of deflection of supporting rolls and other parts and insuring handling the web at high speed without distortion and breaking and with uniform pressing pressures being applied over the width of a very wide web.
A further object of the invention is to provide an improved press mechanism which is capable of removing a greater amount of water from a high speed traveling web and which requires less space for the structure than now necessary with existing press arrangements.
A further object of the invention is to provide an improved method and structure which permits a wet paper mat to be subject to mechanical pressing pressures for an extended period of time wherein the structure is well adapted to predetermination of desired nip length and nip pressures.
A feature of the invention is that the present arrangement employs an extended nip press concept wherein the pressing pressure on the wet web is immediately brought up to a predetermined optimum pressing pressure, and the pressure is maintained substantially constant throughout the length of the extended nip without change and then dropped off at the end of the nip. This arrangement avoids disadvantages of the conventional two roll press nips and of structures which have attempted to extend the length of a press nip, but do so by change in pressure throughout the length of the extended nip such as by having a continual pressure increase through a feed-in compression length and then a decrease in pressure over a decompression length such as taught, for example, by East German Pat. No. 79 919 published Nov. 12, 1971.
Other objects, advantages and features, as well as equivalent structures and methods which are intended to be covered herein, will become more apparent with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments thereof in the specification, claims and drawings, in which: